programs.neovim: write config in $XDG_CONFIG_HOME/init.vim (#1652)

* neovim: write config in $XDG_CONFIG_HOME/init.vim

instead of wrapping the configuration, which has sideeffects
https://github.com/NixOS/nixpkgs/issues/55376

* fix: update test accordingly

tests/modules/programs/neovim/plugin-config.nix

@@ -23,9 +23,8 @@ with lib;
     };
 
     nmt.script = ''
-      vimrc=$(grep -Po "(?<=-u )[^ ]*" < "${
-        builtins.toJSON config.programs.neovim.finalPackage
-      }/bin/nvim")
+      vimrc="$TESTED/home-files/.config/nvim/init.vim"
+      assertFileExists home-files/.config/nvim/init.vim
       # We need to remove the unkown store paths in the config
       TESTED="" assertFileContent \
         <( ${pkgs.perl}/bin/perl -pe "s|\Q$NIX_STORE\E/[a-z0-9]{32}-|$NIX_STORE/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-|g" < "$vimrc"